The separation and enrichment of air by the use of either rate or equilibrium selective adsorbents has been practiced for some time. Nitrogen-selective adsorbents, as typified by ion-exchanged zeolites, are nitrogen-selective at equilibrium and have been used in pressure swing adsorption (PSA) processes. Similarly, carbon molecular sieves (CMS) are used for air separation by PSA processes and rely on a rate selectivity for oxygen. Adsorbents that are oxygen-selective at equilibrium are preferred for many applications since cycle times for PSA processes are not constrained as typically required for rate selective adsorbents. Cyanocobaltates which exhibit oxygen selectivity, for example, have been described in U.S. Pat. Nos. 5,126,466; 5,208,335; 5,141,725; 5,239,090; and 5,294,418.
It has long been known that transition element centers in solid state coordination complexes undergo a reversible interaction with oxygen. Jones, et al. “Synthetic Oxygen Carriers Related to Biological Systems,” Chem. Rev. 79, 139 (1979); Niederhoffer, et al. “Thermodynamics of Oxygen Binding in Natural and Synthetic Dioxygen Complexes,” Chem. Rev. 84, 137 (1984); Bailes and Calvin, “The Oxygen-Carrying Synthetic Compounds. VII. Preparation,” J. Amer. Chem. Soc., 69, 1886 (1947); Adduci, “The Case of Aircraft O2 System based on Metal Chelates,” Chemtech, 575 (1976). Transition element complexes (TECs) are one class of materials known to react reversibily at or below ambient temperatures without breaking the oyxgen-oxygen bond. The use of TECs to selectively remove oxygen from its mixtures with other gases has been disclosed for solutions of TECs, for solid-state TECs or slurries of said solids, for TECs supported on or bound to solid supports and for TECs incorporated in zeolites and for TECs bound chemically to physical supports. Examples of solid state oxygen-selective adsorbents based on discrete TEC units include Co(salen), fluomine, and iron(II) and cobalt(II) complexes of the so-called “picket-fence porphyrin.” Collman, “Synthetic Models for Oxygen-Binding Hemoproteins,” Acc. Chem. Res., 10, 265 (1977).
Each of the known approaches in the art for the use of TECs, however, has been beset by one or more of the following problems: (1) insufficient oxygen capacity, (2) slow reaction rates, (3) decreasing reactivity with time, and (4) a metal ion:oxygen binding ratio of 2:1 (μ-peroxo). Due to these problems, none of these TEC systems has yet been employed in commercially acceptable embodiments for air separation or oxygen removal from gas stream applications.
Extensive literature reports exist describing the reversible oxygenation of TECs having tetradentate ligands, particularly in solution. These materials require an exogenous base (e.g. a molecule or ion, added as a separate component, with a site or sites capable of coordinating to the metal center by electron donation) such as pyridine. The use of an exogenous base is necessary for TECs based on tetradentate ligands in order to provide the five-coordinate deoxy TEC sites required for superoxo binding.
One class of TECs is referred to as “protected” TECs. These use ligand superstructures referred to as “caps,” “picket-fences,” and “bridges” to sterically inhibit μ-peroxo binding and to provide a permanent void on one face of the TEC that serves as an oxygen interaction site. Examples of such ligand systems include porphyrins, cyclidenes, and Schiff bases. Unfortunately, the number, complexity, and yields of the synthetic steps required to make TECs based on these superstructured ligands result in costs that are prohibitively high for many applications. In addition, the high molecular weights inherent in superstructured TECs restrict the oxygen loadings and storages that are achievable. Finally, oxygen interaction rates are slow for known, non-supported solid forms of protected TECs due to intracrystalline diffusion.
More recent reports disclose TECs having tetradentate ligands containing substituents capable of inhibiting μ-peroxo dimer formation in solution, that can be prepared with relative ease and have relatively low molecular weights. The substituents in these systems are typically attached at a single-point. These materials require exogenous donors to provide five-coordinate deoxy TEC sites, and do not show sufficient oxygen uptake in the solid phase for commercial application. U.S. Pat. No. 5,266,283 to Friesen discloses metallo Schiff base complexes which act as regenerable oxygen adsorbents, having a tetradentate structure. These compounds expressly resist dimer formation. However, they lack structural versatility. U.S. Pat. No. 4,451,270 to Roman discloses an oxygen and nitrogen purification process employing a solvent, an “axial base” and an oxygen carrier. The carrier may be a tetradentate metallic compound. However, oxygen uptake in the solid state is not described nor expected.
Reversible oxygenation of TECs having pentadentate ligands in dilute solution is also known. Such disclosures include examples having substituents that inhibit μ-peroxo dimer formation, and wherein the ligand structure and donors are intramolecular. Solid state TECs offer several advantages over those in dilute solution. TECs in solution have problems which have hampered commercial development such as solubility, solvent loss, viscosity, and TEC lifetime. To date, none of the known materials has been found to react reversibly with oxygen in the solid state.
U.S. Pat. No. 5,648,508 to Yaghi and many other publications disclose methods of preparing crystalline or microcrystalline microporous materials using metal and simple ligands that contain cyano, pyridyl, and carbooxylate functional groups. However, this prior art does not teach methods for preparing materials having a metal center that has at least one open coordination position for interaction with substrates. In addition, the simple functional groups taught in this patent, e.g., carboxylates, cannot produce a metal center (e.g., a cobalt center) with appropriate chemical potentials required for chemisorption (reversible oxygenation) and catalytic reactions.
The preparation of coordination polymers based on discrete molecular TECs incorporating sites capable of intermolecular donation has also been described. To date, however, none of these examples has been found to react reversibly with oxygen in the solid state.
The ability of transition element centers in some solid state TECs to undergo reversible interaction with oxygen is known, and the use of supports to disperse or distribute oxygen-selective sites derived from discrete molecular TECs to form oxygen selective adsorbents has been described. Unfortunately, the reported examples where TECs are dispersed on or within a support, within a polymer, or as an integral part of a polymer, contain insufficient oxygen-selective sites for practical use. As an example, Basolo et al. (“Reversible Adsorption of Oxygen on Silica gel Modified by Imidazole-Attached Iron Tetraphenylporphyrin”, J. Amer. Chem. Soc., 1975, 97, 5125-51) developed methods to attach iron porphyrins to silica gel supports via an axial donor. While these demonstrated a substantial improvement in stability relative to solution systems, the TEC content reported was less than 0.1 mol/kg.
Hendricks, in “Separation of Gases via Novel Transition Metal Complexes,” Report Number NSF/ISI87101, Aug. 21, 1987 disclosed attempts to prepare oxygen-selective adsorbents based on TECs by intermolecular donation using peripheral ligand sites. However, it was concluded that the materials tested did not “rapidly and efficiently adsorb oxygen” and that this apparently was due to unfavorable molecular packing.
Another series of materials having oxygen selectivity at equilibrium includes cyanocobaltate materials such as lithium pentacyanocobaltate solvates. U.S. Pat. No. 5,126,466 to Ramprasad et al. discloses solid state cyanocobaltate oxygen-selective adsorbents. The primary ligand, however, is cyanide, which not only poses health issues but also results in a structurally non-versatile product. Further, while gas separation processes which utilize these materials have been disclosed, ranges of composition are restricted, and an ability to optimize performance by adjusting isotherm shapes is limited.
U.S. Pat. No. 6,183,709, assigned to the owner of the present invention, the disclosure of which is incorporated herein, discloses oxygen-selective adsorbent compositions which utilize intermolecular coordination to generate porosity. That invention involves TECs having up to four intramolecular donor ligands coordinated with a transition element ion, wherein the ligands provide a fifth donor site to intermolecularly bond to a second transition element ion contained in a second discrete TEC. These compositions exhibit high oxygen loadings and oxygen half saturation pressures which are suitable for gas separation. In the examples described therein, the structures contain five donors: four donors for intermolecular coordination to the primary metallic center, and one donor for intermolecular coordination with the metal of a second discrete TEC structure. The resultant porosity from this intermolecular coordination offers improved oxygen adsorption characteristics as compared with cyanocobaltate materials of the prior art.
It is among the objects of the present invention to provide a further TEC-based oxygen-selective adsorbent which reversibly binds oxygen, is easily synthesized and has superior porosity.